This invention relates to hot charge-carrier transistors having a base region through which current flow is by hot majority charge-carriers of one conductivity type, and relates particularly but not exclusively to hot electron transistors formed with monocrystalline silicon.
Early experimental forms of hot electron transistors are described on pages 587 to 615 of the book by S. M. Sze entitled "The Physics of Semiconductor Devices", 1969 edition published by Wiley Inter-science. Different transistor structures were considered involving layers of metal and insulator or of semiconductor and metal, but a common feature of all these devices was that the base region was formed by a single sandwiched layer of metal.
U.S. Pat. No. 4,492,971 describes an improved hot electron transistor but still with a metal-based material for the base region. This transistor has an essentially monocrystalline structure comprising a monocrystalline silicon layer forming an emitter on a single thin monocrystalline metal silicide layer which forms the base on a monocrystalline silicon substrate forming the collector. The single metal-silicide base layer is thin (typically 4, 6 or 8 nm) so as to be small compared to the hot electron scattering length in this base material and is made a multiple of the quantum mechanical transmission probability factor. The emitter-base and base-collector junctions are formed by Schottky barriers between the metal silicide base and the silicon emitter and collector.
With the invention of bulk unipolar diodes employing doped semiconductor materials, it became possible to fabricate high performance hot electron transistors and even hot hole transistors having semiconductor base regions. In such diodes the current flow is by majority carriers, but the diode barrier is a doped region in a semiconductor bulk rather than being a barrier at the semiconductor body surface (as is a Schottky barrier). A particularly high quality and efficient bulk unipolar diode is the so-called "camel diode" which is disclosed in U.S. Pat. No. 4,149,174, together with its use in the formation of hot charge-carrier transistors. Other hot charge-carrier transistors using similar bulk unipolar diodes are described in published United Kingdom patent application Nos. 2 056 165, 2 056 166 and 2 118 363.
Thus, from U.S. Pat. No. 4,149,174 and these published United Kingdom patent applications hot charge-carrier transistors are known comprising a semiconductor base region of one conductivity type through which current flow is by hot majority charge-carriers of the one conductivity type. Barrier-forming means forms with the base region an emitter-base barrier serving for injection of the hot charge-carriers of said one conductivity type into the base region. A semiconductor base-collector barrier region is doped with impurity of the opposite conductivity type and is sufficiently thin as to form with both the semiconductor collector and base regions of said one conductivity type a bulk unipolar diode for collecting the hot charge-carriers of said one conductivity type from the base region during operation of the transistor. This permits the achievement of a high collection efficiency for the hot charge-carriers from the semiconductor base, as compared with hot electron transistors having a metal-based base where it is found that quantum mechanical reflections at the abrupt metal-silicon base-collector Schottky interface degrades the collection efficiency.